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The Sierra Nevada Corporation's "Dream Chaser" is one possible contender for NASA's future crew transport to the space station. (Sierra Nevada Corp.)

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What will it take for commercial companies to win a contract to transport NASA astronauts to and from the International Space Station? Andrew Chaikin, author of our December/January 2012 feature story on SpaceX, spoke in mid-November with the head of the agency’s Commercial Crew Program, Ed Mango.

Chaikin: NASA’s basic requirements for commercial crew vehicles include being able to dock with the ISS within 48 hours after launch, remain docked to the station for 210 days with minimal maintenance, and return to Earth within 4 to 8 hours after undocking. What are some of the other requirements?

Mango: Those things are all in the category of performance requirements, which you need to do the mission. Then there are safety requirements, such as the ability to have an abort system that can be operated anytime during ascent. We have requirements for loads on the crew; that’d be G[force]s and vibration and this kind of thing that are health and medical states. There are also requirements for making sure the crew can handle emergencies, such as a depressurization of the capsule. I would say of the requirements, two-thirds to three-quarters are really related to safety, and only about a quarter are related to mission items.

Chaikin: What are the G loads? Can you give me an idea of the max Gs you allow during launch?

Mango: The max Gs is roughly about 3. Similar to shuttle. I think we’re allowing excursions slightly higher than that at certain times, like Max Q [maximum dynamic pressure], but it can’t be sustained. The sustained [allowable G-force] is about 3.

Chaikin: And the probability of “loss of crew” has to be better than 1 in 1000?

Mango: Yes and no. What we've done is we've separated those into what you need for ascent and what you need for entry. For ascent it’s 1 in 500, and independently for entry it’s 1 in 500. We don’t want industry … to [interpret the 1-in-1,000 requirement] to say, “We’ve got a great ascent; we don't need as much descent protection.” In reality we’ve got to protect the life of the crew all the time.

Now [the probability for] the mission itself is 1 in 270. That is an overall number. That’s loss of crew for the entire mission profile, including ascent, on-orbit, and entry. The thing that drives the 1 in 270 is really micrometeorites and orbital debris … whatever things that are in space that you can collide with. So that’s what drops that number down, because you’ve got to look at the 210 days, the fact that your heat shield or something might be exposed to whatever that debris is for that period of time. NASA looks at Loss of Vehicle the same as Loss of Crew.  If the vehicle is damaged and it may not be detected prior to de-orbit, then you have loss of crew.

Chaikin: What about abort? People have said to me that’s the hardest part of commercial crew. Do you agree?

Mango: No, I don't. It is hard, but it's just as hard as guidance and navigation to dock with the station. It's just as hard as mass properties within a capsule or within a winged vehicle, to make sure your center of gravity on your vehicle is right not only for the mission but also for the landing—and when you have a winged vehicle, to make sure that you have the right sink rate and the right control of your spacecraft as it’s entering and landing. Those are just as hard, in fact sometimes harder, because you have less variables you can play with.

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